(1) Field of the Invention
The present invention relates to a method for manufacturing a resin film for a thin film capacitor which is excellent in a heat resistance, a thickness accuracy, a voltage resistance and the like and a resin film for a thin film capacitor.
(2) Description of the Prior Art
A capacitor can be classified into three kinds of a thin film capacitor (or a resin capacitor), a ceramic capacitor and an aluminum electrolysis capacitor according to the kind of dielectric substances. Among three kinds of the above capacitors, the film capacitor has characteristics such as less characteristic change to temperature and a frequency, a high insulation property, a small dielectric loss and the like, and therefore it is considered to be more excellent than other capacitors (refer to a technical document 1).
A resin film for a film capacitor comprises a polypropylene resin (PP resin), a polystyrene resin (PS resin), a polyethylene terephthalate resin (PET resin), a polycarbonate resin (PC resin), a polyvinylidene fluoride resin (PVDF resin), a polyethylene tetrafluoride resin (PTFE resin), a polyimide resin (PI resin), a polyphenylene sulfide resin (PPS resin), a polyethylene naphthalate resin (PEN resin) and the like. It is formed as a dielectric layer for a film capacitor, and a metal deposition layer is formed as an electrode, whereby it is put into practical use.
At present, a film for a film capacitor is a film obtained from four kinds of a PP resin, a PET resin, a PPS resin or a PEN resin, and films comprising other resins have problems in a processing characteristic and a cost and therefore come to be scarcely used (refer to the technical document 1).
However, a film for a film capacitor comprising a PP resin or a PET resin has a problem on a heat resistance due to a use temperature of 105° C. or lower in the PP resin and a use temperature of 125° C. or lower in the PET resin. Accordingly, when it is applied to, for example, a film for a film capacitor in a hybrid car to which a heat resistance of 150° C. or higher is required, (1) a method in which a large-sized cooling device is installed neglecting request to a reduction in a weight or (2) a method in which a film for a film capacitor is installed at a driving seat side and the like far apart from an engine room of a heat source neglecting a space efficiency can not help being employed, and new problems in terms of a reduction in a weight and a cost are brought about.
A film for a film capacitor made of a PPS resin has a use temperature of 160° C. or lower and is provided with a good heat resistance, but it has a low dielectric breakdown voltage and is inferior in a voltage resistance, so that a use range thereof is limited. Further, a film for a film capacitor made of a PEN resin has a use temperature of 160° C. or lower and is provided with a good heat resistance, but it has a large dielectric loss and a large temperature dependability of a dielectric dissipation factor, so that a use range thereof is limited (refer to the technical document 1).
In light of the above limitations, a film made of a polyetherimide resin (PEI resin) attracts attentions as a film for a film capacitor (refer to the technical document 1 and a technical document 2). The above film for a film capacitor made of the polyetherimide resin has a glass transition point of 200° C. or higher and is excellent in a heat resistance and a voltage resistance, and it has a high dielectric breakdown voltage and a small frequency dependability and a small temperature dependability of a dielectric dissipation factor, so that it is most suitable for a film capacitor.
On the other hand, in recent years, a film for a film capacitor is requested to have a thickness of 10 μm or less because of a reduction in a size and a thickness of a film capacitor itself. In addition thereto, a high thickness accuracy thereof is required in order to meet requests of a stable voltage resistance and a uniformization in a winding form. In order to satisfy the above requests, almost all films for a film capacitor are produced by a melt extrusion molding biaxial stretching method.
The melt extrusion molding biaxial stretching method is suited to production of a film for a film capacitor having a thickness of 10 μm or less, but a vertical stretching machine and a lateral stretching machine (tenter method) or a simultaneous biaxial drawing machine has to be used and, therefore processing costs such an electric power cost and the like are likely to go up, and the yields of the raw materials are likely to be deteriorated.
In contrast with this, a non-stretching melt extrusion molding method in which stretching is not carried out after melt extrusion can be reduced in processing costs such an electric power cost and the like, and the high yields of the raw materials can be expected. On the other hand, the problem that a film for a film capacitor having a thickness of 10 μm or less can not be produced at a high thickness accuracy is involved therein.